1. Field of the Invention
The present invention relates to a signal amplifying apparatus, for example, an audio signal amplifying apparatus, which amplifies a signal, for example, an audio signal and supplies the amplified signal to a load, for example, a speaker by way of a cord.
2. Description of the Prior Art
Heretofore there have been proposed a variety of signal amplifying apparatus of this kind.
FIG. 1 shows an example of conventional audio signal amplifiers, which is provided with an amplifier 1. The amplifier 1 has a first input 2 for an audio signal to be amplified, a second input 3 for a feedback signal and an output 4 for an amplified audio signal. The amplifier 1 is further provided with a grounded terminal 5 common to the first and second inputs 2 and 3 and the output 4. The amplifier 1 has the construction of an operational amplifier and derives across the output 4 and the common terminal 5 an output signal of the same polarity as an input signal which is applied across the first input 2 and the common terminal 5 and an output signal reverse in polarity from an input signal which is applied across the second input 3 and the terminal 5.
Furthermore, the amplifying apparatus has a pair of input terminals 6 and 7 across which is supplied an audio signal to be amplified. The input terminal 6 is connected to the first input 2 and grounded via an input register 8. This input terminal 7 is directly grounded.
The amplifying apparatus is further provided with a feedback circuit 9, in which a series circuit of resistors 10 and 11 is connected at one end, as an input, to the output 4 of the amplifier 1 and grounded at the other end and the connection point of the resistors 10 and 11 is connected as an output to the second input 3 of the amplifier 1.
The amplifying apparatus is further provided with a terminal 12 connected with the output 4 of the amplifier 1 and a grounded terminal 13. The terminal 12 is for connection with a cord 17 which is connected with the one terminal 15 of a speaker 14. The terminal 13 is for connection with a cord 18 which is connected with the other terminal 16 of the speaker 14.
According to the apparatus of FIG. 1, when an audio signal is applied across the input terminals 6 and 7, it is supplied across the first input 2 and the common terminal 5 of the amplifier 1, wherein it is amplified and the amplified audio signal is provided across the output 4 and the ground. A portion of the amplified audio signal is fed back across the input 3 and the common terminal 5 of the amplifier 1 via the feedback circuit 9, constituting a first negative feedback loop. Accordingly, a stable audio signal of good quality is obtained between the output 4 of the amplifier 1 and the ground. This amplified audio signal is also obtained across the terminals 12 and 13.
In the apparatus of FIG. 1, in the case where the terminals 12 and 13 are connected with the terminals 15 and 16 of the speaker 14 by way of the cords 17 and 18, respectively, the speaker 14 is driven by the amplified audio signal to produce a stable sound.
FIG. 2 illustrates another example of the conventional audio signal amplifying apparatus. In FIG. 2, the parts corresponding to those in FIG. 1 are identified by the same reference numerals and no detailed description will be repeated. This prior example differs from the example of FIG. 1 in the following points: The resistor 10 constituting the feedback circuit 9 is disconnected from the output 4 of the amplifier 1, that is, the input of the feedback circuit 9 is disconnected from the output 4 of the amplifier 1 and a terminal 21 is provided which is connected with the input of the feedback circuit 9. Along with this, the terminal 7, one end of the input resistor 8 and one end of the resistor 11 forming the feedback circuit 9, that is, a grounding point common to the input and output of the feedback circuit 9, are disconnected from the ground and a terminal 22 is provided which is connected to the terminal 7, one end of the input resistor 8 and the grounding point of the feedback circuit 9. The terminals 21 and 22 are for connection with cords 23 and 24 which are connected with the terminals 15 and 16 of the speaker 14, respectively. Further, a CR parallel circuit 29 of a high-pass capacitor 25 and a resistor 27 is connected between the terminals 12 and 21 and a CR parallel circuit 30 of a high-pass capacitor 26 and a resistor 28 is connected between the terminals 13 and 22.
With the arrangement shown in FIG. 2, an audio signal to be supplied across the input terminals 6 and 7 is provided across the first input 2 and the grounded common terminal 5 of the amplifier 1 via the terminal 22 and the cords 24 and 18. The audio signal is amplified by the amplifier 1, the output from which is derived across the output 4 and the ground. The amplified audio signal is also obtained between the terminals 12 and 13. In the case where the terminals 12 and 13 are connected with the inputs 15 and 16 of the speaker 14 by way of the cords 17 and 18, respectively, the speaker 14 is driven by the amplified audio signal. In the case where the terminals 21 and 22 are connected with the input terminals 15 and 16 of the speaker 14 via the cords 23 and 24, respectively, the output 4 of the amplifier 1 is grounded via the terminal 12, the cords 17 and 23, the terminal 21, the input, the resistors 10 and 11 and the grounding point of the feedback circuit 9, the terminal 22, the cords 24 and 18 and the terminal 13, so that a portion of the amplified audio signal supplied with the speaker 14 is fed back across the input 3 and the terminal 5 of the amplifier 1 via the feedback circuit 9 and cords 17, 18, 23 and 24. Accordingly, there is set up a second negative feedback loop including the feedback circuit 9 and the cords 17, 18, 23 and 24. Further, since the output 4 of the amplifier 1 is grounded via the CR parallel circuit 29 of the capacitor 25 and the resistor 27, the input, resistors 10 and 11 and the grounding point of the feedback circuit 9 and the CR parallel circuit 30 of the capacitor 26 and the resistor 28, a portion of the amplified audio signal from the amplifier 1 is fed back across the input 3 and the terminal 5 of the amplifier 1 via the feedback circuit 9 and the parallel circuits 29 and 30. In consequence, there is established a third negative feedback loop including the feedback circuit 9 and the CR parallel circuits 29 and 30. The capacitor 25 and 26 of the CR parallel circuits 29 and 30 passes on to the third negative loop those unnecessary components contained in the amplified audio signal from the amplifier 1 which have higher frequencies outside of the audio frequency band.
Accordingly, in the apparatus of FIG. 2, an amplified audio signal of good quality, as compared with the amplified audio signal in the case of FIG. 1, is supplied to the speaker 14, producing a sound of good quality as compared with the sound obtainable with the apparatus of FIG. 1.
With the apparatus of FIG. 2, however, in the case where the cords 23 and 24 connected with the terminals 15 and 16 of the speaker 14 are not connected with the terminals 21 and 22, respectively, although the cords 17 and 18 connected with the terminals 15 and 16 of the speaker 14 are connected with the terminals 12 and 13, respectively, the aforesaid second negative feedback loop including the feedback circuit 9 and the cords 17, 18, 23 and 24 is not formed and consequently the negative feedback loop of the amplifier 1 becomes only the third negative feedback loop including the feedback circuit 9 and the CR parallel circuits 29 and 30. This causes a great change in the feedback constant of the negative feedback loop of the amplifier 1 to degrade the frequency-response characteristic of the amplifier 1, resulting in unnecessary low-frequency oscillation being developed or the stability of operation of the amplifier 1 being impaired.